LQQD
Structural
- Apr 14, 2012
- 38
Hi, i´m currently designing a stress ribbon footbridge. I’m having some trouble modelling the dynamic behaviour. I´ll first describe what i have done so far and afterwards the problems I am facing right now.
The footbridge isn´t actually a stress ribbon, it consists of two main cables which support a non-structural precast concrete deck. Width is 3,5m and has a span of 35,0m. Cables have a total area of 49,9cm2 and a young modulus of 160000Mpa. The deck is formed by a 10cm lightweight precast concrete. Precast segments don´t take any longitudinal force or transverse bending moment (moment vector perpendicular to the footbridge axis). The structure could be reasonably represented by two independent cables for a preliminary study.
I am working with SAP2000 V17.3.
So far, the structural FEM model consist of two cables modeled with 2 parallel straight frame objects and the deck modelled with shell elements spanning 3m between cables. Stiffness modifiers are applied to the shell elements in order to work as previously described.
For static load cases the structure responds exactly as by hand calculations for dead weight and live loads.
For the dynamic analysis I first did a modal analysis. Modal load case was defined using the stiffness of the structure deformed by self weights. First frequency is 1,44Hz so, according to various specifications, a more refined calculation should be done in order to restrain maximum accelerations of the deck.
Currently i´m trying to model a walking person over the bridge. I took the recommendation of the BS5400 which models the effect of a pedestrian by applying a moving pulsating load given by:
F(t)=180*sine(2*Pi*f0*t) [N], moving along with a constant speed of v=0,9*f0,
Where f is the fundamental natural frequency, and t is time.
Does anyone knows if F(t) can have a negative value, upward direction, or should it be the absolute value of the sine ? It´s not intuitive to me how a moving person could excerpt an upward force to the deck.
In order to model this i am thinking the following procedure:
1) Defining a moving load with a value of 180N along the centreline of the bridge
2) Define a time history function with the shape of sine(2*Pi*f0*t)
3) Create a Nonlinear (P-Delta + Large displacements) load case with only self-weights acting on the structure
4) Define a multi-step static load case based on the moving load of 180N
5) Create a Nonlinear (P-Delta + Large displacements) time history load case with initial conditions from self-weight load case; this is the load case from point 3), and applying sine function to the multi-step static moving load case of 180N.
So what do you think of this procedure, should it give trustful results? What about time integration methods, which one should give used of gives better results for this kind of structure?
Im kind of new to the forum, would i be violating any rule by rposting the same thread on the bridge and sap2000 forum?
Thanks all!
The footbridge isn´t actually a stress ribbon, it consists of two main cables which support a non-structural precast concrete deck. Width is 3,5m and has a span of 35,0m. Cables have a total area of 49,9cm2 and a young modulus of 160000Mpa. The deck is formed by a 10cm lightweight precast concrete. Precast segments don´t take any longitudinal force or transverse bending moment (moment vector perpendicular to the footbridge axis). The structure could be reasonably represented by two independent cables for a preliminary study.
I am working with SAP2000 V17.3.
So far, the structural FEM model consist of two cables modeled with 2 parallel straight frame objects and the deck modelled with shell elements spanning 3m between cables. Stiffness modifiers are applied to the shell elements in order to work as previously described.
For static load cases the structure responds exactly as by hand calculations for dead weight and live loads.
For the dynamic analysis I first did a modal analysis. Modal load case was defined using the stiffness of the structure deformed by self weights. First frequency is 1,44Hz so, according to various specifications, a more refined calculation should be done in order to restrain maximum accelerations of the deck.
Currently i´m trying to model a walking person over the bridge. I took the recommendation of the BS5400 which models the effect of a pedestrian by applying a moving pulsating load given by:
F(t)=180*sine(2*Pi*f0*t) [N], moving along with a constant speed of v=0,9*f0,
Where f is the fundamental natural frequency, and t is time.
Does anyone knows if F(t) can have a negative value, upward direction, or should it be the absolute value of the sine ? It´s not intuitive to me how a moving person could excerpt an upward force to the deck.
In order to model this i am thinking the following procedure:
1) Defining a moving load with a value of 180N along the centreline of the bridge
2) Define a time history function with the shape of sine(2*Pi*f0*t)
3) Create a Nonlinear (P-Delta + Large displacements) load case with only self-weights acting on the structure
4) Define a multi-step static load case based on the moving load of 180N
5) Create a Nonlinear (P-Delta + Large displacements) time history load case with initial conditions from self-weight load case; this is the load case from point 3), and applying sine function to the multi-step static moving load case of 180N.
So what do you think of this procedure, should it give trustful results? What about time integration methods, which one should give used of gives better results for this kind of structure?
Im kind of new to the forum, would i be violating any rule by rposting the same thread on the bridge and sap2000 forum?
Thanks all!
